This invention relates generally to reducing noise generated when two or more rotating machines are operated and more particularly concerns a method and apparatus for synchronizing the machines by continuously adjusting the speed of one or more of the machines to create a phase relationship between the machines which minimizes noise.
Noises generated when two or more nominally identical rotating machines are running can cause a variety of engineering problems. Perhaps the most common sources of noise problems are propellers, unducted fans (UDF) and turbofans in multi-engine aircraft, although other multiple rotating machinery can create equally troublesome problems. Propeller or turbine noise generated by two-engine or four-engine aircraft results in large noise levels inside the passenger compartment. The major part of this noise is due to propeller or turbine airborne noise radiation transmitted through the fuselage of the aircraft. Small differences in the rotational speed or RPM between each of the engines result in sound waves from each propeller or turbine having different frequencies. The sound waves of different frequencies generate a sound pressure level inside the cabin which is slowly varying with time. This slowly varying sound pressure level is referred to as acoustic beats. Acoustic beats, which are experienced by passengers in virtually all propeller commuter aircraft and in some turbofan aircraft, are annoying and responsible for a high perceived cabin noise level. A similar situation occurs when noise from several rotating machines interferes. Improper phasing results in large perceived sound levels and time varying phasing results in acoustic beats. This problem is common with multiple fan systems used for cooling electrical or electronic equipment.
Most present turboprop engine synchrophasers are only designed to ensure that the speeds of all engines are reasonably close. As a result, engines drift in and out of phase with time. This causes time varying acoustic levels. With more advanced FADEC engines, an attempt is sometimes made to phase-lock the engines which has the effect of preventing acoustic beats. However, these systems do not attempt to minimize noise by feeding back cabin noise levels to the engine speed controller.